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Internet security companies have discovered a third version of the MyDoom e-mail worm circulating on the internet.

MyDoom.C is a modified copy of the virus that ravaged the internet last month. Unlike its predecessor, however, the variant does not use e-mail or the Kazaa peer-to-peer network to spread and is not expected to make much of an impact on the internet, said managed security services provider LURHQ.

MyDoom.C both refines and tames the earlier version of the virus, known as MyDoom.A. Among other changes, the virus fixes problems with the original MyDoom e-mail worm, including errors in the worm's code that made it impossible for many MyDoom-infected machines to launch a programmed denial-of-service (DoS) attack against the SCO Group's website.

Gone also is the expiry date that told machines infected with the original MyDoom virus to stop their DoS attack on 12 February, and instead of depositing a file that opens a backdoor on infected machines, the virus distributes a compressed archive of the worm's original source code.

However, the MyDoom.C author also removed many of the most dangerous features of the original virus, including the highly efficient SMTP engine that enabled infected machines to spew out e-mail messages containing the virus.

That component made the original MyDoom worm the fastest spreading e-mail worm in history, easily defeating Sobig-F, the previous record holder.

Instead, MyDoom.C seeks out and infects machines already infected with the original MyDoom virus by searching for machines listening on port 3127, a telltale sign of MyDoom infection, said security company iDefense.

That approach will give MyDoom.C a solid base of as many as 500,000 machines, but will keep MyDoom.C from spreading much beyond the community of already-infected machines.

The MyDoom.C author also removed a Trojan horse back door, but included a copy of the worm's source code, which is deposited on machines infected with the new variant.

MyDoom.C leaves SCO's website alone, but continues the attack on Microsoft site introduced by MyDoom.B.

The variant does not remove existing versions of the virus and can even run alongside them, said Joe Stewart, senior security researcher at LURHQ.

If started on or between 8 February and 12 February, MyDoom.C- infected machines will launch randomly timed DoS attacks against Microsoft.com. Machines started after 12 February will launch constant attacks against Microsoft's site.

An analysis of the worm's code also uncovered an IP address linked to Ford's web page, www.ford.com, although it is unclear whether the worm targets Ford.

The lack of aggressive spreading features, a staple of most e-mail worms, and the inclusion of the MyDoom.A source code may mean that the MyDoom author is closing shop and handing off his creation to other virus writers to refine, LURHQ said.

"I don't think the internet will shake from this one," said Ian Hameroff, senior security strategist at Computer Associates International.

CA researchers consider the new worm to be a different threat than MyDoom.A, based on a comparison of the two worms' underlying code, and are calling the new threat "DoomJuice", Hameroff said.

While the worm does not pose a risk to Internet users who are not already infected with an earlier version of MyDoom, the wide distribution of the MyDoom. A source could pose a serious risk to the overall security of the internet, Stewart said, adding that uncompiled code could only have come from the MyDoom author and could be useful to less experienced virus writers.

"There's lots of stuff in there - the modified SMTP engine, the spreading algorithm, how MyDoom.A spreads over Kazaa, how it gets e-mail addresses off the hard drive."

Even inexperienced computer programmers could take the code, make small adjustments to it, recompile it and release their own version of MyDoom,.

"The thing I'm most concerned about is, with the source code being available, who's going to take it and what are they going to do with it, " Stewart said. "I think we're going to get copycats on this one."

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